The Photodynamics of Thionated Pyrimidine Nucleobases: Using Time-Resolved Photoelectron Spectroscopy to Explore Characteristics of Excited-State Topographies

The UV photophysics of the pyrimidine nucleobases are surprisingly sensitive to single atom structural modifications. The canonical nucleobases undergo efficient ultrafast internal conversion, and intersystem crossing only becomes relevant on sufficiently long timescales. Thionation, on the other hand, promotes intersystem crossing from the lowest singlet excited state into the triplet manifold with high quantum yields. The subsequent triplet state dynamics uniquely depend on the degree and position of thionation and tautomeric forms of the nucleobases. Here, the electronic population dynamics as observed with gas-phase time-resolved photoelectron spectroscopy are reviewed and placed in context of the underlying potential energy topographies and mechanistic details of the deactivation process.